66 COLLEGE ZOOLOGY 



one from another in structure, physiology, and reproduction. 

 These differences are briefly reviewed in the following para- 

 graphs. 



Morphology. — Protozoa vary in size from the minute blood 

 parasites, such as Plasmodium which causes malaria, to the huge 

 gregarine, Porospora gigantea, which lives in the alimentary 

 canal of the lobster and may be two-thirds of an inch long. 

 Most of them are invisible to the naked eye, and a few are 

 invisible even with the highest powers of the microscope. For 

 example, the organism which is supposed to cause yellow fever 

 is known only by its effects upon human beings, since it has 

 never been seen. 



The shapes of Protozoa are likewise extremely varied. 

 Ameba has no definite shape; many species are globular with 

 radiating projections (Heliozoa, Fig. 18; Radiolaria, Fig. 19) ; 

 Euglena (Fig. 22) is spindle-shaped; Paramecium (Fig. 33) re- 

 sembles a slipper; Vorticella (Fig. 43, C), a bell; Stentor (Fig. 43, 

 A), a trumpet; some like Stylonychia (Fig. 43, B) have definite 

 dorsal and ventral surfaces; in fact, almost every conceivable 

 shape seems to occur in this group. 



Most of the Protozoa are either faintly colored or entirely 

 without pigment. When coloring-matter is present it often con- 

 sists of chlorophyll, or some allied substance, which is contained 

 in chromatophores, e.g. Euglena (Fig. 22, A, chr.). Drinking 

 water is often colored red by Euglena sanguinea, or yellow by 

 Uroglena (Fig. 26) ; the surface of the sea is sometimes colored 

 orange by vast numbers of Xodiluca (Fig. 31), or red by a 

 Dinoflagellate, Peridinium (Fig. 30). 



The simplest kind of locomotor organs are pseudopodia like 

 those of Ameba (Fig. 9, 3). The pseudopodia of some species 

 have a firm axial rod (Heliozoa, Fig. 18), and those of others 

 may branch and fuse with one another (Foraminifera, Fig. 20). 

 Flagella may be likened to very thin pseudopodia that have be- 

 come permanent. They seem to be composed of long fibrils 

 that are spirally wound. Cilia are smaller and more numerous 



